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The channel provides the basic structure of grouping behavior into logical units when communicating over the WebSocket connection. You can think of a channel like a form of controller, but one that's capable of pushing content to the subscriber in addition to simply responding to the subscriber's direct requests.

Channel instances are long-lived. A channel object will be instantiated when the cable consumer becomes a subscriber, and then lives until the consumer disconnects. This may be seconds, minutes, hours, or even days. That means you have to take special care not to do anything silly in a channel that would balloon its memory footprint or whatever. The references are forever, so they won't be released as is normally the case with a controller instance that gets thrown away after every request.

Long-lived channels (and connections) also mean you're responsible for ensuring that the data is fresh. If you hold a reference to a user record, but the name is changed while that reference is held, you may be sending stale data if you don't take precautions to avoid it.

The upside of long-lived channel instances is that you can use instance variables to keep reference to objects that future subscriber requests can interact with. Here's a quick example:

class ChatChannel < ApplicationCable::Channel
  def subscribed
    @room = Chat::Room[params[:room_number]]
  end

  def speak(data)
    @room.speak data, user: current_user
  end
end

The speak action simply uses the Chat::Room object that was created when the channel was first subscribed to by the consumer when that subscriber wants to say something in the room.

Action processing

Unlike subclasses of ActionController::Base, channels do not follow a RESTful constraint form for their actions. Instead, Action Cable operates through a remote-procedure call model. You can declare any public method on the channel (optionally taking a data argument), and this method is automatically exposed as callable to the client.

Example:

class AppearanceChannel < ApplicationCable::Channel
  def subscribed
    @connection_token = generate_connection_token
  end

  def unsubscribed
    current_user.disappear @connection_token
  end

  def appear(data)
    current_user.appear @connection_token, on: data['appearing_on']
  end

  def away
    current_user.away @connection_token
  end

  private
    def generate_connection_token
      SecureRandom.hex(36)
    end
end

In this example, the subscribed and unsubscribed methods are not callable methods, as they were already declared in ActionCable::Channel::Base, but #appear and #away are. #generate_connection_token is also not callable, since it's a private method. You'll see that appear accepts a data parameter, which it then uses as part of its model call. #away does not, since it's simply a trigger action.

Also note that in this example, current_user is available because it was marked as an identifying attribute on the connection. All such identifiers will automatically create a delegation method of the same name on the channel instance.

Rejecting subscription requests

A channel can reject a subscription request in the subscribed callback by invoking the reject method:

class ChatChannel < ApplicationCable::Channel
  def subscribed
    @room = Chat::Room[params[:room_number]]
    reject unless current_user.can_access?(@room)
  end
end

In this example, the subscription will be rejected if the current_user does not have access to the chat room. On the client-side, the Channel#rejected callback will get invoked when the server rejects the subscription request.

Methods
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Included Modules
Attributes
[R] connection
[R] identifier
[R] params
Class Public methods
action_methods()

A list of method names that should be considered actions. This includes all public instance methods on a channel, less any internal methods (defined on Base), adding back in any methods that are internal, but still exist on the class itself.

Returns

  • Set - A set of all methods that should be considered actions.

# File actioncable/lib/action_cable/channel/base.rb, line 114
def action_methods
  @action_methods ||= begin
    # All public instance methods of this class, including ancestors
    methods = (public_instance_methods(true) -
      # Except for public instance methods of Base and its ancestors
      ActionCable::Channel::Base.public_instance_methods(true) +
      # Be sure to include shadowed public instance methods of this class
      public_instance_methods(false)).uniq.map(&:to_s)
    methods.to_set
  end
end
new(connection, identifier, params = {})
# File actioncable/lib/action_cable/channel/base.rb, line 141
def initialize(connection, identifier, params = {})
  @connection = connection
  @identifier = identifier
  @params     = params

  # When a channel is streaming via pubsub, we want to delay the confirmation
  # transmission until pubsub subscription is confirmed.
  #
  # The counter starts at 1 because it's awaiting a call to #subscribe_to_channel
  @defer_subscription_confirmation_counter = Concurrent::AtomicFixnum.new(1)

  @reject_subscription = nil
  @subscription_confirmation_sent = nil

  delegate_connection_identifiers
end
Class Private methods
clear_action_methods!()

::action_methods are cached and there is sometimes need to refresh them. ::clear_action_methods! allows you to do that, so next time you run ::action_methods, they will be recalculated.

# File actioncable/lib/action_cable/channel/base.rb, line 130
def clear_action_methods! # :doc:
  @action_methods = nil
end
method_added(name)

Refresh the cached ::action_methods when a new action_method is added.

# File actioncable/lib/action_cable/channel/base.rb, line 135
def method_added(name) # :doc:
  super
  clear_action_methods!
end
Instance Public methods
perform_action(data)

Extract the action name from the passed data and process it via the channel. The process will ensure that the action requested is a public method on the channel declared by the user (so not one of the callbacks like subscribed).

# File actioncable/lib/action_cable/channel/base.rb, line 161
def perform_action(data)
  action = extract_action(data)

  if processable_action?(action)
    payload = { channel_class: self.class.name, action: action, data: data }
    ActiveSupport::Notifications.instrument("perform_action.action_cable", payload) do
      dispatch_action(action, data)
    end
  else
    logger.error "Unable to process #{action_signature(action, data)}"
  end
end
subscribe_to_channel()

This method is called after subscription has been added to the connection and confirms or rejects the subscription.

# File actioncable/lib/action_cable/channel/base.rb, line 176
def subscribe_to_channel
  run_callbacks :subscribe do
    subscribed
  end

  reject_subscription if subscription_rejected?
  ensure_confirmation_sent
end
Instance Private methods
defer_subscription_confirmation!()
# File actioncable/lib/action_cable/channel/base.rb, line 225
def defer_subscription_confirmation! # :doc:
  @defer_subscription_confirmation_counter.increment
end
defer_subscription_confirmation?()
# File actioncable/lib/action_cable/channel/base.rb, line 229
def defer_subscription_confirmation? # :doc:
  @defer_subscription_confirmation_counter.value > 0
end
ensure_confirmation_sent()
# File actioncable/lib/action_cable/channel/base.rb, line 219
def ensure_confirmation_sent # :doc:
  return if subscription_rejected?
  @defer_subscription_confirmation_counter.decrement
  transmit_subscription_confirmation unless defer_subscription_confirmation?
end
reject()
# File actioncable/lib/action_cable/channel/base.rb, line 237
def reject # :doc:
  @reject_subscription = true
end
subscribed()

Called once a consumer has become a subscriber of the channel. Usually the place to setup any streams you want this channel to be sending to the subscriber.

# File actioncable/lib/action_cable/channel/base.rb, line 196
def subscribed # :doc:
  # Override in subclasses
end
subscription_confirmation_sent?()
# File actioncable/lib/action_cable/channel/base.rb, line 233
def subscription_confirmation_sent? # :doc:
  @subscription_confirmation_sent
end
subscription_rejected?()
# File actioncable/lib/action_cable/channel/base.rb, line 241
def subscription_rejected? # :doc:
  @reject_subscription
end
transmit(data, via: nil)

Transmit a hash of data to the subscriber. The hash will automatically be wrapped in a JSON envelope with the proper channel identifier marked as the recipient.

# File actioncable/lib/action_cable/channel/base.rb, line 208
def transmit(data, via: nil) # :doc:
  status = "#{self.class.name} transmitting #{data.inspect.truncate(300)}"
  status += " (via #{via})" if via
  logger.debug(status)

  payload = { channel_class: self.class.name, data: data, via: via }
  ActiveSupport::Notifications.instrument("transmit.action_cable", payload) do
    connection.transmit identifier: @identifier, message: data
  end
end
unsubscribed()

Called once a consumer has cut its cable connection. Can be used for cleaning up connections or marking users as offline or the like.

# File actioncable/lib/action_cable/channel/base.rb, line 202
def unsubscribed # :doc:
  # Override in subclasses
end